We investigated whether the anatomical modifications following RYGB surgery result in an accelerated transit of inulin to the large intestine and, therefore, a more rapid and pronounced fermentation and production of SCFAs with enhanced secretion of GLP-1 and PYY and beneficial effects on meal-related glycemia and appetite. This was compared to a standard, equicaloric load of MDX. Consistent with previous studies [1, 17, 19, 20], we found that surgery increased postprandial secretion of GLP-1 and PYY; lowered postprandial blood glucose and plasma insulin increments, and reduced postprandial appetite ratings in response to both inulin and MDX. While there was no effect of MDX on breath hydrogen concentrations, the effect of inulin on breath hydrogen was accelerated after surgery with an increase that was earlier in onset (2.5 h vs. 3 h) but less pronounced in magnitude. There was no effect of inulin on plasma SCFAs, nor was there an increase in plasma GLP-1 or PYY concentrations after the snack at 3 h, neither before, nor after surgery. Interestingly, inulin appeared to further potentiate both, the early-phase glucose-lowering and second-meal (3–5 h) appetite-suppressive effects of surgery when compared with MDX. Moreover, there was a strong inverse correlation between early-phase breath hydrogen concentrations (60–180 min) and second-meal “desire to eat” ratings (210–300 min) suggesting a specific and significant physiological interaction with accelerated fermentation of inulin leading to improved appetite-suppression after RYGB surgery

One obvious explanation for inulin to further potentiate the early-phase glucose-lowering effect of surgery in the absence of marked differences in early hormonal responses (GLP-1 and PYY) between inulin and MDX is likely related to the increase in orange juice viscosity at 37 °C body temperature after ingestion (see Supplemental Fig. 1). This may have slowed glucose absorption and thereby secretion of insulin from pancreatic beta cells by increasing the thickness of the unstirred water layer. That the effect was evident only in the first 90 min postprandially but not after the fixed portion snack at 3 h further argues for such an acute mechanism independent of second-meal effects that we had hypothesized. Similar effects of highly viscous fibers (e.g., oat and barley beta-glucans) on meal-related glycemia have been documented before in healthy subjects [21, 22]. The primary mechanism of action has been speculated to involve their ability to increase the viscosity of contents of the upper GI tract and, hence, slow gastric emptying [23]. The rate of gastric emptying is well established to have a substantial impact on postprandial glycemia by determining glucose absorption and incretin hormone secretion [24, 25]. In RYGB patients, however, a slowing of gastric emptying is unlikely to play a significant role given the lack of pyloric control of emptying [18]; moreover, only a fraction of the normal gastric volume can be accommodated due to the greatly reduced gastric lumen. In fact, it has been speculated that the anatomical modifications may result in an almost immediate appearance of particularly liquid meals within 1–2 min in the alimentary limb [1]. Thus, in the absence of proper control of gastric emptying after surgery, chyme viscosity may be a key determinant of postprandial blood glucose and insulin increments. Additional studies with viscous fibers such as grain beta-glucans or pectins are clearly warranted to better understand whether this can be a useful addition to diets in post-bariatric patients.

Several studies further suggest the relevance of the fermentation of prebiotic fibers and the production of SCFAs in glycaemic control and appetite management [13, 26,27,28,29,30,31]. A so-called “second-meal effect” has been described for fermentable fibers to modulate glycaemic responses and appetite not only at the first subsequent meal after consumption but also at later meals on the same or even subsequent days due to colonic fermentation and increased production of SCFA [10, 11]. For example, Cani et al. [27]. demonstrated in normal-weight subjects that 16 g inulin per day consumed over 2 weeks increased breath hydrogen excretion and plasma GLP-1 and PYY concentrations while it lowered hunger rates and postprandial plasma glucose responses after a standardized meal. The molecular mechanism linking SCFA production to the secretion of gut hormones and the control of appetite and glycemia is thought via the activation of G-protein coupled receptors including GPR41 and GPR43 on enteroendocrine cells [30].

Under acute conditions, Wolever and co-workers investigated the effects of inulin and resistant starch on postprandial SCFAs, and gut hormone responses in healthy subjects with overweight and obesity vs. lean subjects [12, 13], an experimental setup which is closer to the protocol used in our study. Overnight-fasted participants consumed 75 g glucose as control or 75 g glucose plus 24 g inulin, or 28.2 g resistant starch, and blood was collected at intervals over 6 h. A standard lunch was served 4 h after the test drink. The authors found that relative to glucose, inulin, but not resistant starch, significantly increased SCFAs from 4–6 h postprandially while neither inulin nor resistant starch affected GLP-1 or PYY. There was no effect of inulin on second-meal glucose and insulin responses in contrast to resistant starch that lowered second-meal glucose and insulin responses at 4–6 h.

Our data are consistent with these findings, we also failed to observe significant effects of inulin on GLP-1 and PYY release or second-meal glycemia following the snack at 3 h. It suggests that prolonged administration with longer adaptation periods to increase colonic fermentation (but not single interventions) may be required for these effects in RYGB patients. We are not aware of any other acute studies that have investigated second-meal effects of prebiotic fibers in patients after RYGB, however, in a recent study in 32 RYGB patients supplemented with probiotic only or probiotic plus inulin for 6 months, patients showed an increase in fasting and postprandial GLP-1 and PYY levels [32].

The early-phase hormonal responses after surgery were comparable between inulin and MDX; both resulted in significant increases in plasma GLP-1 and PYY concentrations at 30 min postprandially when compared with baseline. The relatively moderate increases after surgery are likely related to the small glucose load. It is known that oral protein and lipids typically produce more sustained increases compared to glucose which usually results in monophasic increases [1]. Moreover, the early increase in plasma GLP-1 and PYY with inulin seems controversial given that inulin is not a digestible carbohydrate. However, inulin was administered with orange juice containing 27 g of carbohydrate sugars. Moreover, whether inulin may have directly stimulated some early-phase GLP-1 release independent of its colonic fermentation to SCFAs is unknown. Wölnerhanssen et al. reported that calorie-free polyols such as erythritol and xylitol increase CCK and GLP-1 secretion [33], suggesting that a similar mechanism may account for oligofructose-enriched inulin as used in this study.

With regard to postprandial plasma SCFAs, Wolever and colleagues reported consistent increases in plasma butyrate, propionate, and acetate in response to 24 g of inulin (but not resistant starch) within a 6 h interval in healthy subjects [12]. In our study, we failed to detect increases in plasma lactate, acetate, 2-hydroxybutyrate, and isovalerate, moreover, other organic acids including propionate, and butyrate were not detected consistently despite a comparable dose. Lack of accurate quantification might be one reason for this negative finding although we used a method that was recently validated as a selective and robust LC-MS/MS-based approach to quantitate short-chain carboxylic acids in different human biofluids [14]. However, because only a fraction of colonic-produced SCFAs reach the systemic circulation due to extensive metabolism in colonocytes and liver, SCFA remains difficult to analyze in plasma with even the choice of blood tube that can affect the results. In our study we used EDTA tubes which are not ideal because they can induce acetate contamination [34, 35], however, the collection was consistent across the cohort.

Despite that we failed to detect an increase in plasma SCFAs or plasma GLP-1 and PYY in response to inulin after surgery, inulin appeared to further potentiate the effect of surgery on second-meal (3–5 h) appetite responses, particularly “desire to eat” ratings. Although these differences did not reach statistical significance, they suggest the use of prebiotic fibers to improve post-bariatric dietary management. The underpinning mechanisms, however, require further research. That second-meal “desire to eat” ratings (210–300 min) were inversely correlated with early-phase breath hydrogen concentrations (60–180 min) in the absence of any other significant correlations suggests that accelerated fermentation of inulin may have triggered additional appetite-suppressive effects. However, we failed to detect an effect on plasma GLP-1 and PYY, although other mechanisms may have been involved such as changes in postprandial ghrelin, CCK or leptin secretion, or hepatic metabolism of SCFA, so-called “energostatic” signaling [30, 36]. Also, local increases in active forms of GLP-1 in the splanchnic circulation and subsequent receptor activation may have been involved which we were unable to detect in plasma given that only 10-15% of GLP-1 is estimated to enter the systemic circulation [37].

There are a number of limitations that require consideration when interpreting our findings. First, we only included eight subjects (7 female, 1 male) in this explorative pilot study which limits the full interpretation of the data. Future studies with bigger group sizes may detect fine differences in the dynamics of breath hydrogen, gut hormone secretion, or plasma SCFA. Also, separate studies in both men and women may be warranted given the disparities in glycemic and incretin responses between sexes that have recently been reported [38]. Second, we took plasma samples only up to 5 h postprandially, however, to comprehensively investigate second-meal effects, sampling for longer periods and/or longer-term supplementation of fiber to increase colonic fermentation may be required. Finally, additional measurements of plasma ghrelin or CCK might provide further insights into the underpinning mechanism.

Taken together, we found that RYGB surgery accelerated large intestinal fermentation of inulin indicating faster delivery of inulin to the large intestine. While surgery increased postprandial secretion of GLP-1 and PYY; lowered postprandial blood glucose and plasma insulin increments and reduced postprandial appetite ratings in response to both inulin and MDX, we failed to detect an effect on plasma SCFAs. Moreover, there was no second-meal increase in plasma GLP-1 or PYY concentrations after the snack at 3 h, neither before, nor after surgery. Interestingly, inulin appeared to further potentiate both, the early-phase glucose-lowering and second-meal appetite-suppressive effects of surgery with the latter showing a strong correlation with early-phase breath hydrogen concentrations. This suggests that accelerated fermentation of inulin after RYGB surgery may trigger additional appetite-suppressive effects and accordingly that prebiotic fibers should be further investigated in longer-term studies for its potential addition to diets in post-bariatric patients